The present invention relates to a method of tuning a local oscillator to receive digital broadcasts that employ phase-shift keying (hereinafter, PSK) and orthogonal frequency-division multiplexing (hereinafter, OFDM), and to a digital broadcast receiver using this method.
OFDM broadcasts have multiple subcarrier signals, on which data are transmitted in parallel. In PSK-OFDM broadcasting, each subcarrier signal is modulated by phase-shift keying. The modulated subcarriers are combined and up-converted to the broadcast frequency to create the PSK-OFDM broadcast signal.
Reception of a PSK-OFDM signal requires that a local oscillator in the receiver generate a signal tuned in relation to the broadcast frequency. The local oscillator signal is used to down-convert the received signal to an intermediate-frequency signal, which is then demodulated to obtain the subcarrier phase-shift data. If the local-oscillator frequency is too high or too low, the demodulated phase shifts will be incorrect.
When quadrature phase-shift keying (QPSK) is employed, for example, the phase shifts have nominal radian values of zero, .rho./2, .pi., and -.pi./2. A conventional method of tuning control multiplies the demodulated phase data by four, modulo 2.pi., so that these nominal values all become zero. The sum of the results for all subcarriers is a phase error signal which can be used to control the local oscillator. Correct tuning is maintained by controlling the frequency of the local oscillator so as to reduce the phase error signal to zero.
The conventional method suffers, however, from inherent ambiguity. If the frequency error of the local oscillator causes a phase error of .pi./2, or any integer multiple of .pi./2, multiplication by four will convert the phase error to zero, and the frequency error will go uncorrected. The conventional method is thus able to correct only small frequency errors, such as frequency errors causing phase errors of less than .pi./4 radians.